The way I'd handle the question about whether the mysteries are explained in books would be to say that it's not written about in books. Perhaps is passed down via a magical oral tradition, for instance, or the mystery could be passed on by a heartbeast for House Bjornaer magi.
Just some thoughts...
The Ability Mystery Cult Lore needs to cover the knowledge necessary to administer initiations - which depending on the specific Mystery Cult can be vastly diverse: songs, chants, poems, sagas, philosophical disquisitions and/or the history of the cult are all possible.
There is no need for it to cover initiation scripts proper, which can be passed along by completely different means. Every Mystery Cult made their own decisions there.
Cheers
The books seem to be very ambiguous on this point. In 4ed I would have assumed that these skills could not be learnt from books, but then there was a clear distinction between skills and knowledges, which supported this. This is removed in 5ed, and the only ability I have seen that is not bookable, is heartbeast [HoH:MC]. Thus the RAW assumption must be that other abilities can be learnt from books.
Our SG has decreed that such books are possible, but more or less illegal. The secrets must be protected. You may write one, but the house elders will hunt and/or haunt you. Same for Parma Magica. It is a valid interpretation.
Writing in code sounds like a special skill, which is not covered by RAW. I would have introduced new skills and mechanics if code is desired. That could be handled as a «Secret» Language skill or a secret script (extra point of Artes Liberales). Breaking such a code, I would have said was a season's work and a Artes Liberales roll (to keep it simple). This would be mundane, and not mystic codes.
House Bjornaer could conceivably write house lore books in Gothic. Not secret, but I high threshold for outsiders. It might not even be worthwhile for Clan Ilfetu.
At the end of the day, I doubt that magi Bjornaer or of Verditius would care to write such books. House Bjornaer lore would have too few readers (Clan Ilfetu) only, and Verditius lore can probably be learnt as effectively by experience (exposure) over time. Merinita, on the other hand, might want to write a book to spread the word. Criamon? Well I don't know. Who knows what a criamon thinks, anyway? Parma Magica books is a harder question. They would have readers, and the teaching is explicitly restricted by RAW.
But then again, books which are possible but rare make excellent story hooks. They should not be available at the start of the saga, but they may appear, and they may appear in any kind of obscure language or script. They should remain mysterious for some time even after they are found, and no way should the players be able to anticipate what it takes to read one.
You can self study mystery cult lore, but if you develop your skill exclusively in this way then it is to a new mystery cult you have invented.
Indeed. The big question is what mysteries your cult has access to.
If I understand RAW correctly, a mystic bjornaer may well invent a script to teach Verditius mysteries (or vice versa), effectively reinventing the mystery in a new cult. It may be difficult and risky, but technically it does not seem to be harder to invent scripts and learn mysteries for the wrong cult. The only thing which seems harder is to find a mystagogue to teach or to give you a script.
The TMRE p.18 sympathy bonus awarded by the storyguide "according to how well the rite matches the final Virtue" is critical for creating a working Experimental Script.
So have the bottle of single malt ready, if you wish your Clan Ilfetu Bjornaer to find an initiation to HoH:MC p.134 Bind Magical Creatures on the base of House Bjornaer Lore. 
Cheers
That's a major flaw in the forums system. We need a single malt upload function for PbP sagas.
I am pretty sure though, that the book did not mention the cult lore with respect to the sympathy bonus, only script elements. It is fair, though, to take the cult lore into account.
Historical mystery cults are called Mystery Cults because they are word of mouth and require being invited and initiated to join.
Mystery Cult Lore is a mechanical crutch for both Exoteric and Esoteric Cults. It's a representation of symbolism, codes, and reputation on acquired to join and increase rank. Little of which is likely written down. What will be written down are items which someone without Lore would not interpret as useful to the cult. They won't see the symbolism. Initiation rites without the detailings of Why they work. A script may sometimes have elements to prove only to the mystigogue the initiate is loyal, if satisfied the mystigogue does some secret something to make it work.
For instance the historical Cult of Mithras we have a lot of statues, eating rooms, etc. We have little to no clue what the symbolism means or what they actually believed. Early christian historians wrote the Cult of Mithras was engaging in blasphemous mockery of the Eucharist. The Cult wasn't overly secret as to its existence, it was secret about what and why etc. We have records of people and their ranks, but what they had to do to move up is a mystery.
My interpretation has always been:
Exoteric Mystery Cults are more open to membership and loose with basic knowledge(hence Exoteric). They are larger social groups controlling knowledge with rank. If a person writes down actual secrets there are quite a few that will be able to report the disloyalty. Even though the basic symbolism etc is pretty common. Actual participation in rituals is what is limited and clarification on the true revelation are word of mouth.
Esoteric Mystery Cults will be highly secretive and less available (i.e. esoteric) to uncover knowledge by social action. Esoteric Mystery Cults may write down highly opaque instructions and communication which require word of mouth Lore to understand anything. Or simply may require knowledge of a specific specialty and the cult is measly a gathering of said specialists. There will be few to monitor if they did write something down and few would understand what they wrote. Caution would be advised anyway. However when a person goes years without physically seeing another member, it becomes necessary to write things down. Improving in their ranks will likely be knowledge based, which can be gained outside the Cult.
keep in mind that something written down can still be a secret, if it isn't copied and kept in a secret or secure location for example. Writing a tractatus could easily be a season service for an initiation. Burning a cult lore tractatus might well be a sacrifice for another. If things tie together really well an advanced mystagogue could be required to initiate someone as their "service" in their (presumably self) initiation script, that person would write a tractatus which the mystagogue would read (not having people with a higher lore to learn from) then burn as a sacrifice in his own initiation.
I'm totally stealing that.
AFAICS the rite in question includes not just the game mechanics like Ordeal, Quest, Sacrifice and Travel, but also the songs, processions, dances etc. performed and the stories told. And aren't the latter part of the Mythic Cult Lore?
Cheers
By all means, I agree that this is probably the case for existing scripts, but if you invent a new script, you need to align it with the cult lore you know, not the lores that other use.
But an easy, playable and agreeable solution is to let the sympathy bonus also consider the lore you use, as was suggested by one poster.
Indeed.
This can make a new script far more difficult, if the lore doesn't fit the purpose: think House Bjornaer Lore for Numerology. The storyguide can very well give a sympathy malus for Experimental Scripts, whose rites are based on an inadequate lore.
That poster would just add, that this appears to be in the rules: no need for the troupe to explicitly enable the storyguide for it.
Cheers
Back to the original post - one thing to note for books is the Sanctum Law Insert on page 71 of HoH:TL, particularly with respect to the Exoteric Mysteries.